1. Technical Field
The present invention relates generally to a multiple chip module (MCM), and more particularly, to preparation of an MCM hat having a piston for removal.
2. Related Art
Multiple chip modules (MCMs) have been used in high-performance server systems for many years. Referring to FIG. 1, an example MCM 10 is shown. MCM 10 includes a ceramic substrate 12 that carries a number of chips 14 and other electronic components 16, such as capacitors. An MCM 10 may also include a spar 18. Many MCMs also include a cover, referred to as a hat 30, that may be coupled to the rest of the module by screws and a C-ring seal (neither shown). These hats can be easily removed by unfastening the screws, which allows repair of defective chips on the MCMs. Typically, about 10% of MCMs require repair work. Modules with C-ring sealed hats are expensive because the substrate and C-rings are expensive, and the hats require a ground surface for a base plate. The base plate is used to attach the hat to the substrate using fasteners. The substrate requires a ground surface to minimize stresses on the ceramic after the base plate is secured to the ceramic and the substrate is loaded with 1000 to 1200 lb. force required for compressing the C-ring to achieve a good seal.
With continuing reference to FIG. 1, for the next generation of MCMs, a much less expensive sealing technology has been developed in which an epoxy material 38 will be used to seal hat 30. Epoxy material 38 may include, for example, SylgardÂ® available from DuPont Corporation. With this new sealing technology, repair of MCM 10 requires mechanically shearing off the epoxy material bonded hat. That is, hat 30 is forced (see arrow A) from MCM 10, which can cause damage to electronic components 16 mounted on ceramic substrate 12 because, in many cases, the electrical components 16 (e.g., are higher than an adjacent chip 14. When hat 30 is sheared, a number of components 16 can be removed, necessitating their replacement at significant cost. No processes or mechanisms exist to prevent this damage.
Additional damage from mechanical shear is expected from multiple-part hats, which have been developed to aid thermal dissipation. The illustrative MCM 10 shown in FIG. 1 includes one example of a multiple-part hat 30. Hat 30 includes a base 32 and at least one piston 34. Base 32 includes a sealing ledge 36 for receiving epoxy material 38, which seals hat 30 to MCM 10. Each piston 34 is held in place within base 32 by a piston joint 40. As shown in FIG. 3, in one example, each piston is substantially cylindrical shaped, and each piston joint 40 includes an annulus about a respective piston 34. Piston joint 40 may include any of a variety of materials to hold piston 34 in place relative to base 32. In one example, piston joint 40 includes a solder such as eutectic Sn/Pb, Sn/Ag, Sn/Bi, Sn/Ag/Cu, typically with a solder melting point between 150 Â° C. to 221 Â° C. During set up of MCM 10, hats 30 are reflowed with a biasing force (not shown) to make each piston, e.g., piston 34A shown in FIG. 2, move toward a back surface 42A (FIG. 2) of a corresponding chip 14A to achieve a controlled chip-to-piston gap 46 (FIG. 1). Chip-to-piston gap 46 is filled with a thermal interface material 48 such as advanced thermal compound (ATC) paste, oil or phase change material (PCM). In this fashion, each piston 34 attains a better thermal coupling to a corresponding chip 14 regardless of whether a back surface 42 of chip 14 is parallel with base 32 of hat 30, and regardless of the height of a corresponding chip 14. Chip-to-piston gap 46 may be approximately in the 50 Î¼ m to Î¾ m range for ATC paste, and 3 Î¼ m to 20 Î¼ m for oil or PCM.
Mechanical shear of the above-described multiple-part hat 30 can cause more damage than a non-multiple-part hat. In particular, since a piston 34 may be positioned in a non-planar fashion relative to base 32, piston 34 may move across the top of a corresponding chip 14 and impact a number of adjacent electrical components 16, necessitating replacement of the electrical components. In addition, despite the presence of chip-to-piston gap 46, a piston(s) 34 may also cut into a corresponding chip 14 that has a tilted back surface 42 during mechanical shear of hat 30. As a result, when a piston 34 moves, it may also cause severe damage to a chip(s) 14. Replacement of chips 14 during repair adds significantly to repair costs. Again, no processes or mechanisms exist to prevent this damage.
Damage in the form of undetected microscopic cracks in a chip 14 caused by mechanical shear presents another problem. In particular, a microscopic crack that goes undetected may grow under land grid array (LGA) loading, and cause unexpected device failure during machine life. As a result, the mechanical shear approach to removing a hat may necessitate further repairs and expense.
In view of the foregoing, there is a need for a way to prepare an MCM hat for removal that does not suffer from the problems of the related art.